1. Field of the Invention
The present invention relates to an optical system and an image pickup apparatus having the optical system. In particular, the present invention relates to an optical system and an image pickup apparatus useful in a digital still camera, a video camera, a TV camera, and a silver-halide film camera.
2. Description of the Related Art
As an optical system capable of easily achieving a long back focus and a wide angle of view, a negative lead type optical system is conventionally used. In the negative lead type optical system, a lens unit having a negative refractive power is located at a position closest to the object side. Furthermore, as a negative lead type optical system having a wide angle of view, a fisheye lens or a fisheye zoom lens (hereinafter also referred to as a “fisheye optical system”) is conventionally used.
The fisheye optical system is defined as an optical system that uses a special projection method for utilizing a curved distortion (barrel distortion) to obtain extremely wide-angle images. The fisheye optical system is used, for example, in shooting hemispherical images for meteorological measurement that utilizes the special barrel distortion achieved by the fisheye optical system. Fisheye optical systems enable the capturing of wide-angle images with a convex appearance, by using special mapping projection methods. As typical projection methods used in the fisheye optical system an equisolid angle projection method, in which a solid angle of an object is proportional to the area of an image formed on an image plane, an equidistance projection method, in which the angle of incidence of incident light is proportional to the image height, an orthographic projection method, and a stereographic projection method are used.
As an interchangeable fisheye optical system for a single-lens reflex (SLR) camera, U.S. Pat. No. 6,987,623 discusses a photographic lens having a very wide diagonal angle of view of about 180°, which is achieved by a large amount of negative distortion generated thereby. In general, it is necessary for a fisheye optical system to generate negative distortion. Accordingly, it is necessary to provide a high refractive power to a lens unit having a negative refractive power and disposed in front of the stop or a lens unit having a positive refractive power and disposed behind the stop. In particular, in the fisheye optical system described above, it is necessary to provide a high refractive power to a negative lens unit that is provided on the object side, whose height of incidence of off-axis light flux is high.
In the fisheye optical system, in most cases, a negative lens is located at a position closest to the object side. In the negative lens located at a position closest to the object side, the height of incidence of off-axis light flux and the refractive power of the negative lens are high. Accordingly, a large amount of chromatic aberration (in particular, chromatic aberration of magnification) may generally occur in the negative lens.
In order to prevent chromatic aberration of magnification in the negative lens located at a position closest to the object side, a low dispersion material is generally used as a material of the negative lens. However, if a lens made of a low dispersion material and having a high refractive power is used, it becomes difficult to correct the secondary spectrum.
On the other hand, Japanese Patent Application Laid-Open No. 2008-3108 discusses a fisheye lens made of a low dispersion material and having a low refractive power to correct chromatic aberration of magnification. The zoom lens discussed in U.S. Pat. No. 6,987,623 uses a lens made of a glass material having a high anomalous dispersion for a lens unit located behind the stop. However, in the zoom lens discussed in U.S. Pat. No. 6,987,623, the amount of chromatic aberration may greatly vary during zooming because sufficiently effective achromatism cannot be achieved by each movable lens unit.
On the other hand, the zoom lens discussed in Japanese Patent Application Laid-Open No. 2008-3108 uses a material having a low refractive power and a low dispersion as a material of the negative lens located at a position closest to the object side to correct chromatic aberration of magnification (in particular, the secondary spectrum). However, because the negative lens may have the maximum angle of view θ of about 180°, it becomes difficult to work up the lens. The maximum angle of view θ is expressed by the following expression:θ=Arcsin(d/r)where “r” and “d” denote the radius of curvature and the effective diameter of a specific surface, respectively.
In order to achieve an optical system having a very wide angle of view, capable of effectively correcting chromatic aberration, and having a high optical performance, it is necessary to appropriately set the lens configuration of lens units located in front of and behind the stop and the material of a lens element included in each lens unit. If the lens configuration or the lens material is inappropriately set, it may become difficult to achieve a lens manufactured with a high processing accuracy and having a very wide angle of view of about 180° and a high optical performance.